This invention relates to an acceleration detector and more particularly to an acceleration detector for detecting knocking in an internal combustion engine.
FIG. 1 illustrates one example of an acceleration detector to which the present invention can be applied. This acceleration detector is attached to an internal combustion engine for detecting vibration or knocking of the engine. The acceleration detector comprises a housing 1 defining an annular cavity 2 therein and an acceleration transducer assembly 3 disposed within the cavity 2.
The housing 1 comprises a tubular bushing 4 having a through hole 5 and a flange 6. The housing 1 also comprises a ring-shaped outer case 7 connected to the flange 6 of the bushing 4 so that the cavity 2 is defined therein. The outer case 7 also has a connector 8 radially outwardly extending from the outer case 7 so that an output terminal 9 can extend through the connector 8 for taking out an output signal from the acceleration transducer assembly 3 disposed within the cavity 2. The acceleration transducer assembly 3 includes a washer-shaped plate 10 placed on the flange 6 of the bushing 4, an annular piezoelectric element 11 placed on the washer plate 10, a washer-shaped terminal 12 including a lead 13 connected to the output terminal 9, an annular inertial weight 14 placed on the washer terminal 12 and a threaded ring-shaped stop nut 15 thread engaged with the thread on the tubular bushing 4. An electrically insulating tube 16 is placed on the tubular bushing 4 so that the acceleration transducer assembly 3 is insulated from the bushing 4.
In order to resiliently support and protect the acceleration transducer assembly 3 within the cavity 2 from undesirable environmental conditions, the remaining space of the cavity 2 of the housing 1 which is not occupied by the acceleration transducer assembly 3 is substantially filled with a resilient filler material 17 of a thermo-setting resin. The filler material 17 must be sufficiently resilient after it is cured to allow the movement of the inertial weight 14 relative to the housing 1 when an acceleration is applied to the inertial weight 14 so that the piezoelectric element 11 generates a voltage signal proportional to the pressure exterted on it by the relative movement of the inertial weight 14 against the piezoelectric element 11.
When in use, the acceleration detector is securely mounted on an internal combustion engine (not shown) by a bolt (not shown) inserted into the central through hole 5. the acceleration or the vibration of the internal combustion engine produces the movement of the inertial weight 14 relative to the housing 1, which causes the piezoelectric element 11 to be stressed by the inertial weight 14, whereby an electrical signal indicative of the movement of the inertial weight 14 relative to the engine is generated from the piezoelectric element 11. The electrical signal is provided through the washer terminal 12, the lead 13 and the output terminal 9 to be analyzed to determine as to whether or not a knocking signal which generates upon knocking of the internal combustion engine is involved. When it is determined that a knocking signal is contained in the electrical signal, the operating parameters for operating the engine can be adjusted to increase the output power or decrease the fuel consumption rate.
In the acceleration detector of the above construction, the acceleration transducer assembly 3 is resiliently supported within the cavity 2 by the resilient filler material 17, which is a thermo-cured mixture of a main agent and a curing agent. When the filler material 17 thus cured is soft or sufficiently resilient for allowing the movement of the inertial weight 14 relative to the housing 1 when an acceleration is applied to the inertial weight 14, the acceleration detector exhibits good acceleration detecting characteristics. However, when this detector is exposed to an elevated temperature, the filler material 17 is further hardened to deteriorate the acceleration detecting characteristics. Also, the resilient filler material 17 easily becomes brittle when contacted by gasoline and its resistance to liquid degradation is poor.
When the filler material 17 is made less resilient or sufficiently hard so that a good resistance to environment and a good liquid resistivity are obtained, the acceleration detecting characteristics often become undesirable because of impeded relative movement of the inertial weight. FIG. 2 shows graphs of the detecting characteristics of the related acceleration detector of a general structure as shown in FIG. 1 with and without the cavity 2 filled with the filler material 17. The detection characteristic of the detector when the cavity 2 is not filled with the filler material 17 is shown by a curve A of FIG. 2 from which it is seen that its frequency characteristic is generally smooth and flat. When the cavity 2 is filled with the resilient filler material 17 as shown in FIG. 1, the detection characteristic of the detector has frequency characteristic as shown by an irregular curve B of FIG. 2, from which it is seen that the detection characteristic is disturbed at a lower a frequency band b.sub.1 and is significantly disturbed at a higher frequency region b.sub.2.
Thus, it is highly desirable to obtain an acceleration detector which has flat frequency characteristics and which is yet resistive against the environment.